A known polymerization catalyst for an olefin comprises a combination of a metallocene compound having a group with conjugated pi-electrons, particularly cyclopentadiene and its derivative as ligands and an alkylaluminoxane obtained by the reaction of trialkylaluminum and water. For example, Japanese Patent Application Laid-open No. 19309/1983 discloses a polymerization process of an olefin by the use of a catalyst composed of biscyclopentadienylzirconium dichloride and methylaluminoxane. Furthermore, Japanese Patent Application Laid-open Nos. 130314/1986, 264010/1986, 301704/1989 and 41303/1990 disclose preparation methods of isotactic poly-.alpha.-olefins or syndiotactic poly-.alpha.-olefins and polymerization catalysts which can be used to prepare these poly-.alpha.-olefins having a steric regularity, but the disclosed catalysts all utilize aluminoxanes as co-catalysts.
On the other hand, researches have heretofore been made into an aluminoxane-free homogeneous Ziegler-Natta catalyst system, and it has been known that this kind of catalyst has a polymerization activity for olefins, though this activity is low. The active species of this catalyst is considered to be a cationic metallocene compound or an ion pair type metallocene complex.
In recent years, it has been reported that an isolated cationic metallocene compound having cyclopentadiene or its derivative as a ligand can singly exert the polymerization activity for olefins, even if methylaluminoxane as the co-catalyst does not coexist.
For example, in R. F. Jordan et al., J. Am. Chem. Soc., Vol. 108, 1986, p. 7410-7411, it has been reported that a zirconium cationic complex having tetraphenylborane as an anion and having two cyclopentadienyl groups and a methyl group as ligands can be isolated by utilizing a donor such as tetrahydrofuran as a ligand, and the isolated complex exerts a polymerization activity for ethylene in methylene chloride.
Furthermore, Turner et al. have reported in J. Am. Chem. Soc., Vol. 111, 1989, p. 2728-2729, Japanese Patent Disclosure (Kohyo) Nos. 501950/1989 and 502036/1989 that an ion pair type metallocene complex has a polymerization activity for olefins, and this type of metallocene complex is composed of a metallic compound having a cyclopentadienyl group or its derivative as a ligand containing at least one substituent capable of reacting with a proton and a compound which has a cation capable of donating the proton and which can feed a stable anion. In Zambelli et al., Maclomolecules, Vol. 22, 1989, p. 2186-2189, it has been reported that propylene is polymerized with the aid of a catalyst comprising the combination of a zirconium compound having a derivative of a cyclopentadienyl group as a ligand, trimethylaluminum and fluorodimethylaluminum to obtain an isotactic polypropylene. Also in this case, the active species is considered to be the ion pair type metallocene compound.
In addition, Marks et al. have reported in Langmuir, Vol. 4, No. 5, 1988, p. 1212-1214 that a catalyst supporting a dimethylzirconium complex having a cyclopentadienyl derivative as a ligand on alumina completely dehydrated by a heat treatment at about 1000.degree. C. exerts a polymerization activity for ethylene. This catalyst system is also considered to be a cationic metallocene compound. However, in the connected column of the literature, the description regarding ethylene is seen, but any .alpha.-olefin is not referred to.
The polymerization methods of olefins by the use of a catalyst comprising the combination of the metallocene compound and the alkylaluminoxane disclosed in Japanese Patent Application Laid-open No. 19309/1983 and the like have the feature that the polymerization activity per unit of a transition metal is high. However, in these methods, the polymerization activity per unit of the metallocene compound is high, but the expensive aluminoxane is used in large quantities, and therefore the polymerization activity per unit of the aluminoxane is not so high. In consequence, there is the problem that the production cost of the polymer is high and the other problem that after the polymerization, the removal of the aluminoxane from the produced polymer is very difficult and a large amount of the catalyst residue remains in the polymer.
On the other hand, in the methods of R. F. Jordan et al. and Turner et al., no alkylaluminoxane is used and the cationic zirconium complexes are utilized as the catalysts. Therefore, the above-mentioned problem regarding the alkylaluminoxane is not present. However, the polymerization activity of these catalysts is much lower as compared with that of the catalyst systems using the alkylaluminoxane and most of the complex catalysts cannot polymerize .alpha.-olefins having 3 or more carbon atoms. Furthermore, these methods require a dimethyl complex or the like which can be obtained by alkylating a dichloro complex with an expensive alkylating agent such as methyllithium or a methyl Grignard reagent, and they have the problem regarding the yield of the alkylation, which increases the production cost of the catalysts. In addition, these alkylated metallocene compounds are unstable, and in particular, their solutions in a hydrocarbon solvent or the like easily decompose owing to a trace amount of impurities such as water and oxygen, or light. Accordingly, it is difficult to store the catalysts in the state of the solution, and thus each metallocene solution must be prepared afresh before the polymerization. Moreover, at the time of the polymerization, the contamination of the monomer and the solvent with the impurities must be inhibited to the utmost. In the case that an olefin is polymerized by the use of the Ziegler system catalyst, the impurities can be removed from the monomer and/or the solvent by treating the same with an organometallic compound, particularly an alkylaluminum compound. This removal manner can be applied to the case where the ion pair system catalyst is used, and when the monomer and/or the solvent which has been treated with the alkylaluminum is employed, the polymerization activity of the catalyst for the olefin is improved to some extent, but nevertheless, this activity is poorer as compared with the combined catalyst system using the alkylaluminoxane as the co-catalyst. In the method of Zambelli et al., propylene is polymerized with the aid of the catalyst system comprising trimethylaluminum, dimethylaluminum fluoride and the zirconium complex to obtain an isotactic polypropylene, as described above, and the expensive aluminoxane and the dimethyl complex are not used. However, in this kind of catalyst, the organometallic compound containing fluorine is used and the polymerization activity is extremely low. This polymerization activity can be slightly improved by using a halogenated hydrocarbon such as methylene chloride as the reaction solvent, but nevertheless it is still low, and the halogenated hydrocarbon, since being poisonous, is not preferable as the solvent.